1. Field of the Invention
This invention relates to a video signal reproducing apparatus and more particularly, to an apparatus arranged to reproduce a composite signal which is formed by superimposing plural kinds of pilot signals of different frequencies upon a video signal one after another from recording tracks formed by the composite signal on a recording medium.
2. Description of the Prior Art
For a video tape recorder of the kind having rotating heads arranged to record and/or reproduce a video signal on a video tape such as a magnetic recording tape (hereinafter will be called VTR for short), tracking methods have been proposed for accurately tracing the recording tracks of video signal in carrying out reproduction thereof. Such prior art methods include a first method in which a control signal is recorded along the edge of the video tape for tracking control and a second method in which four kinds of pilot signals of different frequencies are recorded beforehand by superimposing them on a video signal. The details of the second method are as described below:
In the accompanying drawings, FIG. 1 shows a recording pattern on a video tape. The illustration of FIG. 1 includes a video tape 10; an area 11 provided for obtaining a reproduced video signal; overlapped recording areas 12 and 13 in which the video signal is overlappingly recorded; a first recording/reproducing head 14 (hereinafter will be called A head); and a second recording/reproducing head 15 (hereinafter will be called B head). The recording/reproducing heads 14 and 15 have different azimuth angles from each other. Video signal recording tracks A1-A7 are recorded by the A head 14 while video signal recording tracks B1-B6 are recorded by the B head 15. One of pilot signals (hereinafter will be called pilot signals F1, F2, F3 and F4) of four different frequencies, f1, f2, f3 and f4, are recorded in each of the recording tracks, that is, one for every field period of the video signal, by superimposing them upon the video signal.
In reproducing the video signals thus recorded, a pilot signal reproduced by the reproducing head is multiplied by a reference signal of frequency which is substantially the same as that of a pilot signal superimposed on a recording track which is mainly traced (hereinafter will be called the main track) by the reproducing head. Then, crosstalk components from adjacent tracks located in front and in rear of the main track are separated from a multiplication output thus obtained. The levels of these two crosstalk components are compared with each other to obtain a tracking control signal corresponding to the positional error of the reproducing head relative to the main track.
This method enables tracking control signals to be obtained throughout the video signal recording track and thus ensures accurate tracking even in the case of a curved video signal recording track. Further when the reproducing head is mounted on an electric-to-mechanical converting element such as a bi-morph element to make its rotating face shiftable in a direction intersecting the video signal track, accurate tracking also can be accomplished even when the tape is to be moved for reproduction at a speed different from the speed at which recording is performed. In that instance, however, the multiplication must be performed by accurately determining a reference signal having the same frequency as that of the pilot signal recorded in the main track. For example, when a reproducing operation is to be performed on the recorded tape of FIG. 1 at a reproducing speed three times as high as the recording speed, the reproducing operation is performed as follows: After the record of the track A1 (having pilot signal of frequency f1) is reproduced by the A head, the track B2 (having pilot signal of frequency f3) is reproduced by the B head. Then, the A head reproduces the track A4 (f4) and the B head the track B5 (f2) and the tracks are thus reproduced one after another. The frequency of the reference signal to be used for the multiplication changes for every field period of the video signal in such a manner as f1 - f3 - f4 - f2 - f1 - f3 ---, which differs from the sequence of frequency rotation f1 - f2 - f4 - f3 of the pilot signals with which recording is accomplished. This can be understood from the following table showing frequency rotations which take place during reproducing operations carried out at varied speeds integral numbers times as high as the recording speed:
TABLE 1 ______________________________________ Frequency shifting rotation for reproduction at tape speeds which are integral numbers times as high as recording tape speed K Frequency rotation ______________________________________ 4n f1 f2, f2 f4, f4 f3, or f3 f1 4n + 1 f1 .fwdarw. f2 .fwdarw. f4 .fwdarw. f3 .fwdarw. f1 .fwdarw. ... 4n + 2 f1 f2, f2 f4, f4 f3, or f3 f1 4n + 3 f1 .fwdarw. f3 .fwdarw. f4 .fwdarw. f2 .fwdarw. f1 .fwdarw. f3 ... ______________________________________ Notes: K: The multiplying rate of reproducing tape speed relative to tape speed adopted for recording n: An integer
While the above table shows the frequency rotation for reproducing operations carried out at tape speeds integral numbers times as high as the tape speed used for recording, the frequency rotation becomes more complex for reproducing operations to be carried out at tape speeds increased at multiplying rates other than those obtained by multiplying the recording speed by integers. Especially, in the case where a tape speed is to be changed during a reproducing operation, it has been impossible to determine the frequency of the reference signal to be used and thus, tracking control has been hardly possible
FIG. 2 is a circuit diagram showing a circuit arranged to effect the rotation of the frequencies of reference signals to be generated. FIG. 3 is a timing chart showing the timing of the operation of the circuit of FIG. 2.
Referring to FIG. 2, a magnetic head 21 is arranged to detect the position of a video head of a cylinder 20 on which the rotating video head is mounted. The output of the magnetic head 21 is delayed by a monostable multivibrator 22. The output is then processed by a flip-flop (hereinafter will be called FF) 23 and is changed into a head switchover pulse signal SEA, which corresponds to the signal known by the name of 30 PG. This signal SEA is then supplied to a data selector 25. Meanwhile, the signal SEA is also frequency divided by an FF 24 and is supplied to the data selector 25 as another selected input SEB. The data selector operates as shown in the following table:
TABLE 2 ______________________________________ Truth values of data selector 25 Selected input Data input SEA SEB f1 f4 f2 f3 Reference signal produced ______________________________________ H L H -- -- -- f1 H H -- H -- -- f4 L L -- -- H -- f2 L H -- -- -- H f3 ______________________________________ Notes: H: High level L: Low level
As shown in Table 2 and FIG. 3, the frequency of the reference signal produced changes in synchronism with the timing of the head switch-over and becomes f4 when both the signals SEA and SEB are produced at a high level (hereinafter will be called H); becomes f3 when the signal SEA is at a low level (hereinafter will be called L) while the signal SEB is at H; becomes f1 when the signal SEA is at H while the signal SEB is at L; and becomes f2 when both of the signals SEA and SEB are at L. However, this arrangement does not enable the apparatus to shift the rotation of frequency according to a varied tape speed set at the time of reproduction. The tape speed at which reproduction can be accomplished thus has been limited to speeds 4n+1 times as high as the tape speed used for recording.